Usually, a piece of photographic equipment, such as a camera, a cell phone configured with a photographing function, etc., is able to implement auto-focusing.
Auto-focusing is achieved by utilization of light reflection. Specifically, light reflected by a target object (i.e. an object being photographed) is caught by an imaging sensor (such as, a CCD sensor, a CMOS sensor, etc.) of the photographic equipment. Information corresponding to the reflected light is further processed by a computer for actuating a power focus apparatus, so as to achieve auto-focusing.
Generally, an auto-focus apparatus may include: at least one imaging lens, at least one focusing lens, at least one photosensitive component, and at least one focal-length adjusting mechanism.
According to their basic principles, auto-focus methods are divided into two categories: active auto-focus method and passive auto-focus method. Active auto-focus method achieves auto-focusing by measuring a distance between the lens and the target object. Passive auto-focus method achieves auto-focusing by detecting the definition of an image on a focusing screen.
Specifically, when the active auto-focus method is used for achieving auto-focusing, the photographic equipment is generally configured with an infrared generator (or an ultrasound generator). Infrared light (or ultrasonic sound wave) generated by the infrared generator (or the ultrasound generator) reaches the target object and is reflected by the target object. The reflected infrared light (or the reflected ultrasonic sound wave) is captured by a receiver in the photographic equipment, and thus auto-focusing is achieved based on the received light (or sound wave). Optical principle employed by the active auto-focus method is similar to that of a triangulation method, based on which the distance between the lens and the target object is calculated. The active auto-focus method may be employed by low-grade cameras (e.g. various eye-level finders) for implementing auto-focusing.
However, the active auto-focus method has following disadvantages.
On one hand, the infrared light from the infrared generator or the ultrasonic sound wave from the ultrasound generator may be reflected by the target object towards various directions, or may not reach the target object, that means limited amount of reflected infrared light or reflected ultrasonic sound wave can be captured by the receiver. Especially when the target object has a slanted surface or a smooth surface, auto-focusing performance of the photographic equipment may be degraded. Further, when the target object has high brightness and is far away from the photographic equipment, it may be difficult to achieve auto-focusing.
On the other hand, as the photographic equipment is configured to actively send light (e.g. infrared light) to the target object, low contrast between the light actively sent and light received may exist. Thus, it may be hard to achieve auto-focusing when the ambient light is weak. When the target object is in narrow strip form or in motion, light may be absorbed by the target object. Furthermore, when the target object is behind a piece of glass, the light should penetrate through the glass before reaching the target object. Accordingly, the light may be reflected by the glass, and focusing on the target object may be hard to achieve.
Regarding the passive auto-focus method, auto-focusing is implemented by receiving and analyzing ambient light reflected by the target object.
In comparison with the active auto-focus method, the passive auto-focus method possesses following advantages. On one hand, there is no need to have an emitting system (e.g. infrared generator, ultrasound generator, etc.) in the photographic device, thus both power consumption and size thereof may be reduced. Therefore, the passive auto-focus method can be used in small handheld devices, such as a cell phone, etc. On the other hand, even when the target object has high brightness, or the target object is in backlight, far away from the photographic equipment, or behind a piece of glass, etc., ideal focusing performance can be achieved.
However, the passive auto-focus method has following disadvantages. Firstly, when the target object is in narrow strip form and has weak texture, false focusing is easy to occur and focusing time is long. Secondly, under circumstance of low contrast and weak ambient light, focusing performance is also poor. Thirdly, when the target object is in motion, includes polarized light, has black color, or has a mirror surface, etc., it is hard to achieve auto-focusing.
Accordingly, neither existing auto-focus apparatuses nor existing auto-focus methods can achieve good focusing performance, thus imaging performance of the photographic equipment configured with the existing auto-focus apparatus is degraded.